The present invention relates to a wireless communication system, and more particularly, to a wireless communication system for a high-speed object moving on a fixed path.
Currently, in preparation for a mobile traffic explosion which may not be solved by third-generation and fourth-generation mobile communication capacities, fifth-generation mobile communication technology needs to be developed. Fifth-generation mobile communication provides high capacity using a wider frequency band in order to provide improved performance and capacity as compared to a conventional mobile communication scheme and provides an optimal structure for communication between a base station and a user equipment (UE) in consideration of available frequency band. In addition, coverage can be supplemented and enlarged using heterogeneous network technology composed of a macro-cell and a small-cell and data processing capacity and speed can be increased using multiple bands.
In order to control increased data traffic, device-to-device (D2D) technology for performing direct communication between devices without via a network or machine-to-machine (M2M)/Internet of Things (IoT) technology for connecting peripheral things via a network to deliver or obtain necessary information at anytime and anywhere have become important.
Fifth-generation mobile communication provides high capacity using a wider frequency band in order to provide improved performance and capacity as compared to a conventional mobile communication scheme and provides a communication structure between a base station and a user equipment (UE) in consideration of available frequency band.
It is possible to enlarge coverage and increase data capacity using heterogeneous network technology, to reduce network traffic using D2D technology and to improve service quality using M2M technology.
A 4G cellular communication system which is currently being discussed is designed based on one basic frame and is designed to provide optimized performance to a user who moves at a low speed. Such a system is designed to support a user who moves at a speed of 350 km/h, but the performance thereof is inferior to that of a user who moves at a low speed. If such a cellular communication system is applied to a high-speed train, link quality between a network and the high-speed train is deteriorated and sufficient link capacity cannot be ensured due to high mobility of 350 km/h. If the speed of the high-speed train will exceed 500 km/h in the future due to technological development, performance deterioration will become more serious and quality of a wireless data service provided to a passenger will be significantly decreased. If a scenario in which a high-speed train uses some capacity of a macro base station (BS) is used, data communication of other users in a cell may be deteriorated.
Accordingly, instead of wireless communication, wired communication may be used for communication between a network and a high-speed train. For example, communication between a high-speed train and a network may be performed through an AC signal using a railroad, with which the high-speed train is in contact. However, in such a system, capacity of the railroad is low and it is difficult to establish more links due to physical restriction that the number of simultaneously connected railroads is restricted to two. Similarly, a power line communication (PLC) method of performing communication using a power line may be used. However, this method has the same disadvantages as the communication method using the railroad and cannot be disadvantageously applied to a train without a power line.
A conventional mobile terminal is portable and compact, but has relatively low performance. However, as digital communication apparatuses have been increasingly used and services have been diversified, demand for seamless high-speed data transmission at anytime and anywhere has increased. Thus, demand for high-speed data transmission in high-speed moving objects such as cars, trains or aircrafts has increased.
A conventional cellular system was designed to operate at the movement speed (e.g., 120 km/h) of a general vehicle, but was mainly optimized for a low-speed mobile terminal, such that a data transfer rate is low and network connection is barely maintained at a predetermined speed or more. In addition, since the conventional cellular system is designed in consideration of high frequency selectivity, pilot overhead is significantly high. If such a system is applied to a very high speed (e.g., 350 km/h or more) of a high-speed train without change, quality of a link between a network and the high-speed train may deteriorate. As a problem of the conventional communication technology, an open-loop transmission and reception scheme should be applied to a high-speed mobile terminal. While a closed-loop transmission and reception scheme is far superior to an open-loop transmission and reception scheme in terms of performance in a low-speed environment, the open-loop transmission and reception scheme is far superior to the closed-loop transmission and reception scheme in terms of performance in a high-speed environment. The closed-loop transmission and reception scheme is based on accurate channel estimation according to the position of a UE. However, at a high speed, channel estimation is difficult due to Doppler shift according to the speed and overhead for channel information feedback rapidly increases due to a time-variant channel property. Accordingly, in a high-speed environment, better performance can be obtained by improving spatial diversity via an open-loop transmission and reception scheme which does not require channel estimation except for link quality (or channel quality) estimation for determining a modulation/demodulation level.